Monthly Mechanic: Cable-Stayed Bridges

All the bridge types I’ve covered so far originate in nature. They are simple, based around a single load-carrying element: a beam, an arch, a cable.

Beam, arch, cable – elementary forms.

Not so for the cable-stayed bridge. While it may superficially resemble a suspension bridge – two or more tall towers with roughly triangular arrays of cables between them – a cable-stayed bridge has no main cable. Instead, all the little cables that support the deck run directly to the nearest tower. While this load path might seem more efficient (less steps!), keep in mind that you’re using diagonal cables to hold a vertical load.

Suspension Bridge load path vs. Cable-Stayed Bridge load path

Since a cable only works in tension, a diagonal cable always supports a diagonal force. On paper you can break a diagonal force into horizontal and vertical components. If you’re analyzing a cable stay, the vertical force component is the weight of a portion of deck, which is usually the same for every cable. As you move from near the tower to midway between towers, the cable’s angle gets farther and farther from vertical, which means the horizontal force component gets bigger and bigger. You need a big cable to support a big force… and that’s why the cable stays on a cable-stayed bridge are much thicker than the suspenders on a suspension bridge.

As the angle from vertical grows, so does the force in the cable, even for equal vertical loads.

So it turns out cable-stayed bridges aren’t all that efficient in terms of cost per foot of span. (Although in windy places they are safer than suspension bridges, being less susceptible to vibration.) In most cases they’re chosen as much for their aesthetics as for their ability to get from point A to point B.

All cable-stayed bridges could be described as iconic. Here are the ones I’m featuring pictures of: the Zakim Bridge in Boston (which crosses the Charles River, although it’s hard to tell with all the infrastructure surrounding it including three other bridges within 200 feet), the Øresund Bridge (which connects Denmark to Sweden over the Baltic Sea), and the Russky Bridge in Vladivostok (at 3622 feet the world’s longest cable-stayed span).